1. Field of the Invention
The present invention relates to a cathode for a lithium-ion secondary battery having a lithium-ion intercalation/de-intercalation ability, a lithium-ion secondary battery, a vehicle and a power storage system equipped with the battery.
2. Description of Related Art
Recently, there is an increasing demand for prevention of global warming and concern about depletion of fossil fuels. Much attention has been paid to the development of an electric automobile consuming less energy for travelling and a power generation system using natural energies such as solar light and wind power. However, the above mentioned technologies have the following problems, which prevent such technologies from being utilized widely.
A problem with an electric automobile is that a driving battery thereof has a low energy density, resulting in a short travelling distance after one time charging. On the other hand, a problem with a power generation system using natural energies is that the system requires a large capacity of battery for leveling a fluctuation of generated output due to the large fluctuation of power generating capacity. This may result in requiring a higher cost. Accordingly, in both technologies, it is demanded to develop an inexpensive secondary battery having a high energy density.
A lithium-ion secondary battery has a higher energy density per unit weight than such second batteries as a nickel-metal hydride battery and a lead battery. Herein, the lithium-ion secondary battery is expected to be applied to an electric automobile and a power storage system. For the application, the lithium-ion secondary battery needs to have a higher energy density to meet increasing demands for an electric automobile and a power generation system. The increase in the energy densities of the cathode and anode of the battery is required in order to increase the battery energy.
Here, Li2MO3—LiM′O2 solid solution is expected as a cathode active material with a high energy density. Note that M is at least one element selected from the group of Mn, Ti and Zr, and M′ is at least one element selected from the group of Ni, Co, Mn, Fe, Ti, Zr, Al, Mg, Cr and V. Hereinafter, Li2MO3—LiM′O2 solid solution is referred to as a solid solution based cathode active material.
A solid solution of Li2MO3 having a layered structure and electrochemical inactivity and LiM′O2 having a layered structure and electrochemical activity is a cathode active substance with a high capacity. The solid solution may have an electric power capacity more than 200 mAh/g activated by charging a lithium metal with a voltage more than 4.4 V when initial charging is applied (see Journal of the American Chemical Society, 128 (26), pp. 8694-8698 (2006)).
A cathode for a lithium-ion secondary battery disclosed in Japanese Laid-Open Patent Publication Nos. 11-144734 and 2009-146811, includes an oxygen absorbing substance, by which oxygen generated from the cathode if the battery is overcharged or at a high temperature is absorbed.
Further, in Japanese Laid-Open Patent Publication No. 11-144734, a lithium-ion secondary battery is disclosed, in which an oxygen absorbing substance consisting of a metal oxide is fixed to a conductive material to absorb the oxygen released from the cathode at a high temperature.
Japanese Laid-Open Patent Publication No. 2009-146811 discloses a material such as LiMoO2 having a lithium-ion absorbing and releasing ability and an oxygen absorbing ability if the battery is overcharged is used as an oxygen absorbing substance, so as to prevent an energy density from lowering caused by the addition of the oxygen absorbing substance.
Further, Japanese Laid-Open Patent Publication No. 2009-76446 discloses a cathode for a lithium-ion secondary battery, comprising a cathode active material coated with vanadium oxide to improve conductivity of the solid solution based cathode active material.
Here, it should be noted that oxygen is released from a cathode when a solid solution based cathode active material is activated with charging on a voltage more than 4.4V described in Journal of the American Chemical Society, 128 (26), pp. 8694-8698 (2006). The released oxygen may cause a problem to destroy an electron conductive network in a cathode by reacting with a conductive material in the cathode. Further, an oxygen gas thus vaporized may increase an internal pressure in the battery to break a battery container. Moreover, the oxygen may react with an electrolyte solution, which results in lowering the capacity.
In the cathode of Japanese Laid-Open Patent Publication No. 11-144734, the oxygen absorbing substance does not have a lithium-ion absorbing and desorbing ability, resulting in significant decrease of the energy density per weight of the cathode. Further, since the oxygen absorbing substance is fixed to the conductive material, the absorbing efficiency of oxygen thereof is poor.
In the cathode of Japanese Laid-Open Patent Publication No. 2009-146811, the oxygen absorbing substance after absorbing oxygen does not have a sufficient lithium-ion absorbing and desorbing ability in a range of the general operating voltage (2-5V for lithium metal). Accordingly, this may also decrease the energy density when the solid solution based cathode active material releasing oxygen when initial charging is applied is used.
In the cathode of Japanese Laid-Open Patent Publication No. 2009-76446, vanadium oxide (VOx[2≦x<2.5], and V2O5) used for coating a cathode active material does not have a lithium-ion absorbing and desorbing ability and an oxygen absorbing ability simultaneously. Herein, a VOX based material does not have a lithium-ion absorbing and desorbing ability, and V2O5 has a poor oxygen absorbing ability, resulting in a lower energy density of the cathode.